The manufacture of semiconductor devices involves the performance of a series of process steps using a variety of high tech production and metrology tools in a certain order and often within a certain period of time. The primary function of a wafer logistics system in a semiconductor fabrication facility, or “fab,” is to deliver the wafers to each of the tools at the right time, as well as to track the location and status of the wafers throughout the process.
Automated material handling systems (“AMHS”) are used in fabs to carry out the automated functions more efficiently, consistently, and safely than can be done via manual means. Process and tool changes have placed additional demands on the AMHS. Such demands include the necessity for cross-floor and cross-phase transportation and increased transportation volume, the combination of which often results in traffic jams, delays, and lower tool utilization.
When a wafer carrier, such as a standard mechanical interface (SMIF) pod or front opening unified pod (“FOUP”) containing wafers is to be transferred, a manufacturing execution system (“MES”) determines to what destination in the fab the wafer carrier should be transferred. Once the destination decision has been made, the MES sends a transfer request to a material control system (“MCS”), which calculates a detailed transportation route using a real time dispatching (RTD) system and then notifies a transfer manager to execute the transfer step-by-step. AMHS's have been used extensively in the semiconductor fabrication field.
FIG. 1 is a diagram of a portion of a conventional automated materials handling system (AMHS) 100. The typical system includes a plurality of bays (rows) of storage areas. Each bay has a stocker 140, 141, which includes bins for holding a plurality of wafer carriers 170-172, such as standard mechanical interface (SMIF) containers for loading 200 mm (8 inch) wafers, or front opening unified pods (FOUPs), which may be used to load 300 mm (12 inch) wafers. The stockers 140, 141 hold the SMIFs or FOUPs 170-172 in preparation for transporting a SMIF or FOUP to the loadport of a processing tool 130, 131.
An overhead hoist transport (OHT) 110, 112 associated with each bay transports the SMIF or FOUP with wafers from a respective stocker 140 or 141 to a loadport of its respective tool 130 or 131, for processing in one of the tools (fabrication process machines). The OHTs 110, 112 have a plurality of intra-bay overhead transport (OHT) vehicles 160, 164 for transporting wafer carriers 170, 171 between tools 130, 131 and their respective local stockers 140, 141. An inter-bay OHT 111 is provided between stockers 140, 141. Additional inter-bay overhead transport (OHT) vehicles 161-163 are provided for transporting wafer carriers 170, 171 between stockers 140, 141.
Because the availability of wafers to be processed at the time the equipment is ready to perform the processing has a major impact on the overall production rate, it is important to operate the AMHS 100 in a manner that supplies wafers quickly as soon as they are needed (i.e., as soon as the tool is ready to receive the next wafer to be processed). A frequently used measure of the AMHS performance is the Operator Service Time (OST). The OST is an efficiency index of the AMHS 100 that measures the period of time between issuance of a retrieval command for a lot of wafers (by the load port of the processing tool) and the time when the wafers are available to the operator at the tool. One significant component of the OST is the tool load port time, which is the period between issuance of a retrieval command by the load port of the processing tool 130 or 131 and the time when the wafers are transferred to the load port of the equipment.
To reduce the latency between the issuance of the retrieval command and the transfer of the wafers to the load port, an overhead hoist buffer (OHB) 150 was set up to provide a temporary storage function in AMHS 100. The OHB 150 is physically closer to the downstream tool 131 than the local stocker 141. Also, transferring a wafer carrier from OHB 150 to an OHT vehicle 164 takes less time than transferring the wafer carrier from a storage slot within stocker 141 to the vehicle 164 (which also includes time to transfer the wafer carrier internally, to the output port of the stocker). FIG. 2 is a flow chart showing the operation of the OHB 150.
At step 200, a lot is tracked out from upstream tool 130. The wafer lot has finished process in upstream tool 130 and is ready to go to the next process in tool 131.
At step 202, the dispatch rule determines the next destination tool.
At step 203, if tool 131 is available, and this lot is the next one to be processed by the downstream tool 131, then step 210 is performed next, and the lot is transferred directly to the loadport 131a of the next tool 131. If tool 131 is not available, (e.g., if there is another wafer lot ready to be used by downstream tool before this wafer lot), step 204 is performed next.
At step 204, this wafer lot is sent to the local stocker 141 for the next tool 131.
At step 206, a system watchdog function (not shown) periodically checks whether the OHB 150 is empty. When the system watchdog determines that the OHB 150 is empty and that the local stocker 141 has a wafer lot ready to be processed by tool 131, then step 208 is performed.
At step 208, the system watchdog triggers the OHT 112 to transfer this lot from local stocker 141 to OHB 150.
At step 210, when the load port 131a of the tool 131 is available, the wafer carrier is transferred from the OHB 150 to the load port 131.
Using the prior art system and method, if the OHB 150 is already filled, then it cannot accept another wafer carrier. This may reduce overall wafer lot performance of tool 131.